The invention pertains to the machining of axially symmetrical parts such as transmission shafts, as well as crankshafts and camshafts.
The processing of these parts is generally accomplished in several stages and on various types of machining equipment. For example, in the case of crankshafts, the arms and pins of each main bearing typically are first rough machined by external circular milling or internal circular milling. Since the pin diameter, in particular, will not normally meet roundness, runout, and surface quality specifications after this step, it must subsequently undergo finish machining, hence turning or grinding, before being completed.
However, there are also special machining processes, especially for crankshafts, which make it possible to achieve such high precision in a single machining step that further intermediate machining processes are not required prior to heat treatment. This is often the case with so-called round broaching.
It has proven advantageous, both when machining a workpiece in several steps on several different machines and when performing finished machining in a single process, to use machine tools with two opposing main spindles, each of which is also equipped with jaw chucks which thus clamp both ends of the workpiece in the jaw chucks during machining. In contrast to a purely pointed pickup, as typically used between lathe centers, such two-ended clamping in jaw chucks has the advantage that radial deflection of workpieces, particularly those with low flexural stiffness, occurs considerably less frequently. This has a favorable effect on dimensional accuracy, especially when the central portion of the workpieces are being machined. Such a machine tool is identified as the type K 10 crankshaft lathe disclosed by Boehringer Brothers, in its 1955 brochure No. 2000555.
However, the clamping of both ends of the workpiece in jaw chucks has a disadvantage in that machining of the peripheral surface in the end regions of the workpiece held in the jaw chucks, and possibly the machining of the adjacent arm and/or flange facings, must be completed on other machines and with a different method of mounting before the actual machining of the piece itself.
Such rough machining of the pins in the end regions is done, for example, on combination centering lathes, with the workpieces rotating, in order to machine the pin diameter. Inasmuch as the crankshafts are centered by counterbalancing, yet another machine is required to machine the peripheral surfaces and arm surfaces in the end regions of the crankshaft. As a rule, even these different types of mounts, and the clamping and unclamping of the workpiece they require, are enough to result in compromised dimensional accuracy in the finished machined piece. In particular, the trueness of the peripheral surfaces with reference to the lathe centers leaves sonething to be desired, because no better solution is possible even with a stationary workpiece and a rotating tool.
A further disadvantage consists in the large amounts of unproductive time taken up with machining using differing processes and in various mounts, with transporting the workpieces back and forth among the individual machines, and with intermediate storage of the workpieces in buffer storage areas between machining steps, after which they must be re-mounted and aligned. Obviously, to mount them, the main drives of the machine in question must be brought to a stop, which not only leads to additional unproductive time but also wastes energy and increases wear on the drive in question.
Aside from this, having several machines and any necessary linkages between them requires much higher investments in space and machinery than does a single machine tool that allows the complete machinery of such a workpiece.